Measuring instrument



Sept. 19, 1939. w. R. KOCH MEASURING INSTRUMENT WERNA 1 I l/infield R.

r o. SEER m.

Zhmentor Koch Patented Sept. 19, 1939 UNITED STATES MEASURING INSTRUDIENT Winfield R. Koch, Merchantville, N. .L, assignorto Radio Corporation of. America, a corporation of Delaware Application June 22, 1937, Serial No. 149,619

9 Claims.

This invention relates to measuring instruments such as are useful in indicating the distortion of radio signals due to selective fading or the like, and has for its principal object the pro- 5 vision of an improved device and method of operation whereby the periods during which the side band voltage exceeds that of the carrier is readily determined both as to the amount of the excess side band voltage and the time of its du- 10 ration.

Another object of the invention is the provision of an improved means of correctly indicating variations in the carrier wave voltage.

A further object is the provision of improved 15 means for separately indicating or recording the relation between the carrier and side band voltages.

Previous measuring devices of this character have not been altogether satisfactory in operagg tion for the reason that they give a false indication when the side band peaks exceed those of the carrier and do not make available sufficient information with respect to the character and duration of the signal distortion. In

25 accordance with the present invention, this difficulty is avoided by (1) closely regulating the volume and carrier frequency of the signal, (2 sharply separating the side band and carrier components, (3) detecting these separated com- 30 ponents and (4) balancing the detected component voltages against one another. As hereinafter described in greater detail, this method affords a ready means of indicating both the relation between the carrier and side band volt- 35 ages and the variations of the carrier voltage.

In the exemplified embodiment of the invention, the carrier voltage component is also utilized to prevent variation in the volume and carrier frequency of the received signal.

40 The invention will be betterunderstood from the following description when considered in connection with the accompanying drawing and its scope is indicated by the appended claims.

The single figure of the drawing is a wiring 4 diagram of a preferred embodiment of the invention.

This form of the invention includes an antenna it from which signals are transmitted through a radio frequency amplifier II and a 5 first detector l2 toa pair of highly selective channels having as their respective first stages an intermediate frequency amplifier l3 and an intermediate frequency amplifier I i. The channel following the amplifier i3 is hereinafter des- 55 ignated as the side band channel and channel folowing the amplifier I4 is similarly designated as the carrier channel. .The separation of the side band and carrier components is effected by sharply tuned filters which may be of the piezo electric, magneto-strictive or other suitable types.

Thus coupled to theoutput circuit of the intermediate frequency amplifier, I3 is a bridge network filter l'5 which is balanced only at carrier frequencyxand similarly coupled to the output circuit of the amplifier I4 is a bridge network filter it which is balanced at frequencies other than that of the carrier, the balancing frequency of the filter I5 being dependent on the resonant frequency of a piezo electric crystal device ll and the balancing frequencies of the filter i6 being dependent on the resonant frequency of a piezo electric device 18.

The side band voltage component output of the filter l5 isapplied through'an amplifier i9 andv a detector 20 tothe input circuit of a direct current amplifier 2| which is normally biased to cut ofi" and which includes in its output circuit a signal distortion indicator 22. The indicator 22 is'a voltmeter or recording voltmeter 25 whose deflection depends upon the degree of distortion due to fading of the carrier with respect to the side bands, as will appear subsequently.

Similarly coupled to the output circuit of the intermediate frequency amplifier M through the filter network l6 are a carrier frequency amplifier 23 and detector-"24, the output voltage of which is applied through a direct current amplifier 25 to a carrier fading indicator 28. The indicator 26 is a zerol-center voltmeter, or recording voltmeter. It. is connected between two normally equi-potential points 38 and 49 so that its deviation measures a change in the direct current voltage drop across the plate load re- 4 sistor 50 of tube 25.

It will be observed (1) that the anode circuits of the side band detector 2!] and carrier detector Z iinclude respectively a resistor .21 and a r sistor 28, (2) that the potential drop of the resister 28 is applied to the-control grid of the device 25, and (3) that the difference between the potential drops :of the resistors 27 and 28 is applied to the control grid of the device 2|. Under these conditions, the device 26 is actuated only in response to variation in the carrier voltage and the device 22 is actuated only in response to an excess of the side band voltage over that of the carrier. It is of course apparent that by not biasing the device 2i to cut oil? a continuous indication of the difference between the carrier and side band voltages may be derived at the device 22. Automatic volume control potentials for the amplifier H and detector I2 may also be derived from thedetector resistor 28 as indicated by the leads 29 and 30.

To ensure accurate functioning of the improved measuring instrument,it is desirable that the intermediate frequency be maintained with in narrow limits for the reason that the filter networks I5 and I6 must be very sharply tuned to separate the side band and carrier component's. To this end there is coupled through a capacitor 3| to the input circuit of the carrier detector 24 an automatic frequency control channel which includes an amplifier 32, a bridge network filter 33 of the piezo electric type, an amplifier 34, detectors 3536 and an automatic frequency control tube 31 arranged to regulate the frequency of an oscillation generator 38 which applies the heterodyne frequency to a control grid of the detector [2.

In the operation of this'automatic frequency control channel, the carrier is applied to an amplifier tube 32; thence through a coupling transformer to the bridge circuit 33 including a quartz crystal 39 inone arm of the bridge as shown. Part of the bridge voltage is also applied in phase to the cathode and plate elec trodes, respectively, of the two diodes 35-36 through series blocking condensers 40-4I. The output of the bridge is amplifiedby the amplifier 34 and applied to the plate and cathode electrodes, respectively, of the diodes 35 and 36 in push-pull relation through the transformer l5. The circuit is so adjusted that at resonance approximately equal voltages will occur. between each of the two electrodes of each diode and ground.

The bridge circuit is balanced for all frequencies except in the region? where the crystal 39 is resonant and at the resonant frequency maximum output voltage is obtained, the phase shift occurring in the bridge'being 90. Strictly speaking, maximum voltage output from the bridge is secured not at the trueresonant frequency of the crystal, but at a slightly higher frequency, where the equivalent reactance of the crystal is inductive and sufiiciently large to form a series resonant circuit with inherent tube and wiring capacities connected to the output terminal of the bridge 33. The 90 phase relation between the voltage applied to any series resonant circuit and the voltage drop across one of the reactances forming thev resonant circuit is, of course, a well recognized fact.

An approximately 90 phase relation will there fore exist between the voltages applied between each of the two electrodes and ground, in each diode 35 and 36. Because of'the voltage equality and the phase relation, the rectified voltage output from each diode will be the same in amplitude. However, with the circuit shown, one diode output is positive with respect to ground while that of the other is negative. With a high resistance, 42, bridged across the two output resistors 46 and 41, the center tap 43 will therefore be at ground potential and no A. F. C. voltage will be applied to the control grid of the device 31.

For frequencies slightly off this frequency, the 90 phase relation does not occur, but instead the phase angle is more or less than 90, depending on which side of the desired frequency the frequency of the applied carrier falls. With any angle except 90, the outputs of the two diodes are unequal. If the voltages applied between each of the two electrodes of one diode and ground have less than a 90 phase relation, the voltage across the diode is reduced. Thus, with a phase angle of 0 degrees, and equal voltages, the voltage across the diode would be nil. Because of the degree relation in one of the voltages applied to the other diode, secured by means of the center tapped secondary of transformer 45 the phase angle of the voltages between each electrode of this diode and ground becomes more than 90 as the phase angle of the voltages on the first diode becomes less than 90. The voltage across the diode therefore becomes greater than with the 90 degree phase relation, and the output increases. Thus, when the voltages applied between each electrode and ground become 180 out of phase with each other twice the voltage will occur across the diode. This will occur at the same frequency that causes 0 phase relation and zero output for the other diode.

Therefore, when the signal is slightly mis-tuned in one direction, the output of one diode will be larger than that of the other, and when the signal is mis-tuned in the other direction, the output of the one diode will be smaller than that of the other. The center tap 43 of the bridging resistor 42 is therefore either positive or negative with respect to ground depending on the direction of mis-tuning. By application of this voltage to a frequency control tube, any mis-tuning produces a controlvoltage tending to change the frequency of the heterodyne oscillator in a direction such as to bring the carrier more nearly in resonance with the crystal. In order to secure the desired phase relation between the plate and grid potentials of the automatic frequency control amplifier 31, a capacitor 44 having a high reactance compared to the resistance of the grid leak, should be connected between the plate and grid of this amplifier to produce phase shift of approximately 90 in the phase of the grid voltage. Any change in bias on this tube will change the mutual conductance of this tube, and therefore change the amount of plate current from this tube through the coil in the plate circuit. Because this current will be approximately 90 out of phase with the voltage between plate and ground, the tube ,will affect the circuit in much the same manner as would a reactance, and variation of grid bias will change the effective reactance, thus producing a change in the tuning of the oscillator. In this manner, any small initial mis-tuning or subsequent drift of the oscillator would be compensated.

I claim as my invention:

1. The combination of means receptive to radio frequency signals having carrier and side band components, means for segregating the side band and carrier components of said signals, means for rectifying said components, and means for indicating a change in the relative amplitudes of said rectified components.

2. The combination of means receptive to radio frequency signals having carrier and side band components, means for segregating the side band and carrier components of said signals, means for rectifying said components, means for indicating a change in the relative amplitudes of said rectified components, and means for indicating a change in the amplitude of said carrier component.

3. The combination of means receptive to radio 75 frequency signals having carrier and side band components, means for amplifying said signals, means for segregating the side band and carrier components of said amplified signals, means for rectifying said components, means responsive to the amplitude of said rectified carrier component for regulating the gain of said amplifying means, means for indicating a change in the amplitude of said rectified carrier component, means for combining said rectified components, and means responsive to the difierence of said combined components for indicating a change in the relative amplitudes of said rectified components.

4. A device of the character described in claim 1 in which a pair of piezo-electric filter networks are utilized for segregating the side band and carrier components of said signals.

5. The combination of means receptive to radio frequency signals having carrier and side band components, means for segregating the side band and carrier components of said signals, means for rectifying said components to obtain a first and a second direct current potential proportional respectively of the amplitudes of said side band and carrier components, means for combining said first and second potentials to obtain a difference potential, and means responsive to said difference potential for indicating when the amplitude of said first potential exceeds that of said second potential.

6. In a distortion indicator, the combination including means receptive to radio frequency signals having carrier and side band components, means for maintaining the amplitude and frequency of said radio frequency signals at a predetermined value, means for segregating the side band and carrier components of said signals, means for rectifying said components, and means for indicating a change in the relative amplitudes of said rectified components from a predetermined value.

'7. In a distortion indicator, the combination including means receptive to radio frequency signals having carrier and side band components, means for maintaining the amplitude and frequency of said radio frequency signals at a predetermined value, means for segregating the side band and carrier components of said signals, means for rectifying said components to obtain direct current potentials corresponding to the respective amplitudes of said components, means for subtracting said direct current potentials, and means operable when one of said direct current potentials exceeds the other of said direct current potentials for indicating distortion due to a change in the relative amplitudes of said carrier and side band components.

8. A device of the character described in claim '7, which is further characterized by means operable by a change in the amplitude of said carrier component for indicating the degree and direction of said change.

9. In a device of the character described, a frequency control system which comprises a carrier current Whose frequency is to be controlled, a network unbalanced at the desired frequency of said current, means for applying said current to said network to obtain an output voltage whose phase varies with the frequency of said carrier current, a pair of diode rectifiers having plate and cathode electrodes, means for impressing said output voltage in phase opposition on the plate electrode of one and the cathode electrode of the other of said rectifiers, means for impressing on the remaining electrode of each of said rectifiers a voltage in quadrature phase with the voltage on said plate and cathode electrodes, means for combining currents resulting from said applied voltages to obtain a resultant current whose direction is a function of the frequency of said carrier current, and means for utilizing said resultant current to maintain the frequency of said carrier current constant.

WIN'FIELD R. KOCH. 

